The major function of the epidermis is to provide a barrier between the external environment and the organism. To fulfill this function keratinocytes undergo a complex pathway of differentiation, culminating in keratinocyte cornification and in the formation of extracellular lipid-enriched lamellar membranes in the stratum corneum. Despite its great importance, the regulation of keratinocyte differentiation and the factors that allow for the coordinate formation of the corneocytes and extracellular lipid lamellar membranes are largely unknown. Recently it has been recognized that endogenous intracellular lipids activate PPARs and LXR. The ability of these nuclear hormones to sense cellular lipid levels, and then to regulate gene expression has led to the recognition that these receptors are "liposensors". We postulated that activation of these nuclear hormone receptor "liposensors", by the lipids generated during the formation of lamellar bodies and/or by metabolic products of these lipids, could provide a mechanism that coordinately regulates the formation of the corneocytes ("bricks") and extracellular lipid membranes ("mortar") during epidermal differentiation (Lipids->Activate Liposensors->Stimulate Expression of Proteins Required for the Cornified Envelope). As a first step in proving this hypothesis we have shown that activators of PPARs and LXR stimulate epidermal differentiation. In this proposal, we plan to focus on the molecular mechanisms by which activators of PPARs and LXR stimulate differentiation. Hypothesis: PPAR alpha, delta, gamma, and LXR activators stimulate keratinocyte differentiation by increasing the levels of AP-1 proteins, thereby stimulating the expression of a variety of key genes required for keratinocyte differentiation that contain AP-1 regulatory elements in their promoters. Aim 1. To determine if the stimulation of differentiation induced by PPAR delta and PPAR gamma activators is receptor mediated (Studies have already been completed for PPAR alpha and LXR activators during the current funding cycle). Aim 2. To determine if PPAR and LXR activation of AP-1 response elements is a general mechanism accounting for the increased expression of genes important in differentiation. Aim 3. To determine if activators of PPAR alpha, delta, gamma, and LXR increase AP-1 protein levels thereby increasing AP-1 activity and the functional importance of these increases. Aim 4. To determine the mechanism by which activation of PPARs and LXR increase AP-1 protein and mRNA levels we will localize the sites in the promoters of the AP-1 proteins that mediate the increase in expression induced by PPAR alpha, delta, gamma, and LXR activators.